Apparatus and methods for high-speed drivers

ABSTRACT

Apparatus and methods for high-speed drivers are provided herein. In certain embodiments, a high-speed driver multiplexes two or more data streams. The high-speed driver is implemented with a mux-then-driver topology that provides multiplexing in a predriver circuit. Thus, the multiplexer is eliminated from the full rate output path to relax timing. Driver amplitude control schemes are also disclosed in which a controllable driver includes a group of differential series source transistor (SST) driver slices that are connected in parallel with one another to drive a pair of output terminals, and a group of attenuator slices that are connected in parallel with one another across the pair of output terminals. Additionally, the controllable driver includes a control circuit that activates an attenuator slice for each SST driver slice that is decommissioned to provide output amplitude control.

FIELD OF THE DISCLOSURE

Embodiments of the invention relate to electronics, and moreparticularly, to driver circuits for high-speed communications.

BACKGROUND

Serializer/deserializer (SerDes) systems can be used in a variety ofapplications such as telecommunications, optical networks, and/orchip-to-chip communication.

A SerDes system includes a serializer that converts two or more parallelinput data streams into a high-speed serial data stream, and adeserializer that converts the high-speed serial data stream into two ormore parallel output data streams of reduced speed. Thus, datatransmission can be provided over a reduced number of lines to lower pincount.

SUMMARY OF THE DISCLOSURE

Apparatus and methods for high-speed drivers are provided herein. Incertain embodiments, a high-speed driver multiplexes two or more datastreams. The high-speed driver is implemented with a mux-then-drivertopology that provides multiplexing in a predriver circuit. Thus, themultiplexer is eliminated from the full rate output path to relaxtiming. Driver amplitude control schemes are also disclosed in which acontrollable driver includes a group of differential series sourcetransistor (SST) driver slices that are connected in parallel with oneanother to drive a pair of output terminals, and a group of attenuatorslices that are connected in parallel with one another across the pairof output terminals. Additionally, the controllable driver includes acontrol circuit that activates an attenuator slice for each SST driverslice that is decommissioned to provide output amplitude control.

In one aspect, a driver circuit includes an output terminal configuredto provide an output data stream, and a first driver subcircuitconfigured to receive a first input data stream of a lower bit rate thanthe output data stream, and to drive the output terminal based on thefirst input data stream in response to a transition of a first clocksignal. The first driver subcircuit includes a first driver transistorconnected between a high supply voltage and the output terminal with noother transistors therebetween, a second driver transistor connectedbetween a low supply voltage and the output terminal with no othertransistors therebetween, a first pull-up predriver circuit configuredto control the first driver transistor, and a first pull-down predriverconfigured to control the second driver transistor.

In another aspect, a serializer/deserializer (SerDes) system includes adeserializer, and a serializer including a driver. The driver includesan output terminal configured to provide an output data stream to thedeserializer, and a first driver subcircuit configured to receive afirst input data stream of a lower bit rate than the output data stream,and to drive the output terminal based on the first input data stream inresponse to a transition of a first clock signal. The first driversubcircuit includes a first driver transistor connected between a highsupply voltage and the output terminal with no other transistorstherebetween, a second driver transistor connected between a low supplyvoltage and the output terminal with no other transistors therebetween,a first pull-up predriver circuit configured to control the first drivertransistor, and a first pull-down predriver configured to control thesecond driver transistor.

In another aspect, a method of multiplexing data streams is provided.The method includes providing an output data stream on an outputterminal, receiving a first input data stream of a lower bit rate thanthe output data stream as an input to a first driver subcircuit, anddriving the output terminal based on the first input data stream inresponse to a transition of a first clock signal using the first driversubcircuit, including controlling a first driver transistor connectedbetween a high supply voltage and the output terminal with no othertransistors therebetween using a first pull-up predriver, andcontrolling a second driver transistor connected between a low supplyvoltage and the output terminal with no other transistors therebetweenusing a first pull-down predriver.

In another aspect, a driver circuit includes a pair of output terminalsconfigured to provide a differential output signal, a plurality ofdifferential series source transistor (SST) driver slices electricallyconnected in parallel with one another and configured to drive the pairof output terminals, a plurality of attenuator slices connected inparallel with one another across the pair of output terminals, and acontrol circuit configured to selectively deactivate one or more of thedifferential SST driver slices to control an amplitude of thedifferential output signal, and to enable a corresponding number of theattenuator slices to provide output impedance compensation.

In another aspect, a method of output swing control in a driver circuitis provided. The method includes providing a differential output signalon a pair of output terminals, driving the pair of output terminalsusing a plurality of differential series source transistor (SST) driverslices electrically connected in parallel with one another, deactivatingone or more of the differential SST driver slices to control anamplitude of the differential output signal, and enabling acorresponding number of a plurality of attenuator slices to provideoutput impedance compensation, wherein the plurality of attenuatorslices are connected in parallel with one another across the pair ofoutput terminals.

In another aspect, a serializer/deserializer (SerDes) system includes adeserializer, and a serializer comprising a driver including a pair ofoutput terminals configured to provide a differential output signal tothe deserializer, a plurality of differential series source transistor(SST) driver slices electrically connected in parallel with one anotherand configured to drive the pair of output terminals, a plurality ofattenuator slices connected in parallel with one another across the pairof output terminals, and a control circuit configured to selectivelydeactivate one or more of the differential SST driver slices to controlan amplitude of the differential output signal, and to enable acorresponding number of the attenuator slices to provide outputimpedance compensation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of one embodiment of a multiplexingdriver.

FIG. 1B is one example of a timing diagram for the multiplexing driverof FIG. 1A.

FIG. 2A is a schematic diagram of another embodiment of a multiplexingdriver.

FIG. 2B is a schematic diagram of another embodiment of a multiplexingdriver.

FIG. 3A is a schematic diagram of one embodiment of a driver halfcircuit for a multiplexing driver.

FIG. 3B is one example of a timing diagram for the driver half circuitof FIG. 3A.

FIG. 4A is a schematic diagram of another embodiment of a multiplexingdriver.

FIG. 4B is one example of a timing diagram for the multiplexing driverof FIG. 4A.

FIG. 5A is a schematic diagram of another embodiment of a multiplexingdriver.

FIG. 5B is one example of a timing diagram for the multiplexing driverof FIG. 5A.

FIG. 6 is a schematic diagram of another embodiment of a multiplexingdriver.

FIG. 7A is a schematic diagram of one embodiment of a pull-up predrivercircuit for a multiplexing driver.

FIG. 7B is one example of a timing diagram for the pull-up predrivercircuit of FIG. 7A.

FIG. 8A is a schematic diagram of one embodiment of a driver quartercircuit for a multiplexing driver.

FIG. 8B is one example of a timing diagram for the driver quartercircuit of FIG. 8A.

FIG. 9 is a schematic diagram of one embodiment of a driver withcontrollable swing and constant output impedance.

FIG. 10A is a schematic diagram of another embodiment of a driver withcontrollable swing and constant output impedance.

FIG. 10B is a circuit diagram of a portion of the driver of FIG. 10A.

FIG. 11 is a graph of one example of driver amplitude reduction versusattenuation setting for the driver of FIGS. 10A and 10B.

FIG. 12A is a graph of one example comparison of driver output swingversus attenuation setting for two implementations of drivers.

FIG. 12B is a graph of one example comparison of driver current versusattenuation setting for two implementations of drivers.

FIG. 12C is a graph of one example comparison of driver output impedanceversus attenuation setting for two implementations of drivers.

FIG. 13 is a schematic diagram of one embodiment of a SerDes system.

DETAILED DESCRIPTION OF EMBODIMENTS

The following detailed description of embodiments presents variousdescriptions of specific embodiments of the invention. However, theinvention can be embodied in a multitude of different ways. In thisdescription, reference is made to the drawings where like referencenumerals may indicate identical or functionally similar elements. Itwill be understood that elements illustrated in the figures are notnecessarily drawn to scale. Moreover, it will be understood that certainembodiments can include more elements than illustrated in a drawingand/or a subset of the elements illustrated in a drawing. Further, someembodiments can incorporate any suitable combination of features fromtwo or more drawings.

A SerDes system can include a serializer that generates a high-speedserial data stream based on data streams associated with different timeoffsets or delays. For instance, in a half rate SerDes system, aserializer can combine a first half rate data stream and a second halfrate data stream into a full rate data stream having a bit period, withthe second half rate data stream delayed by the bit period relative tothe first half rate data stream.

In certain embodiments herein, high-speed drivers for multiplexing twoor more data streams are provided. The high-speed driver is implementedwith a mux-then-driver topology that provides multiplexing in apredriver circuit. Thus, the multiplexer is eliminated from the fullrate output path to relax timing. For example, in an implementation withtwo data streams, a timing constraint is relaxed by a factor of two.

Moreover, implementing the multiplexer in the predriver reduces a sizeof the multiplexing transistors relative to an implementation in whichan explicit T-gate multiplexer is included along the output signal path.By reducing the size of the multiplexing transistors, the multiplexersize is shrunk and the total capacitance is reduced to achieve reducedpower and a tighter output eye diagram.

FIG. 1A is a schematic diagram of one embodiment of a multiplexingdriver 10. FIG. 1B is one example of a timing diagram for themultiplexing driver 10 of FIG. 1A.

With reference to FIG. 1A, the multiplexing driver 10 operates tomultiplex an odd half rate data stream D_(ODD) (also referred to hereinas an odd data stream) and an even half rate data stream D_(EVEN) (alsoreferred to herein as an even data stream) to generate a full rate datastream D_(OUT). Thus, the multiplexing driver 10 provides two-wayinterleaving.

The multiplexing driver 10 receives the odd data stream D_(ODD) and theeven data stream D_(EVEN), as well as a first clock signal CK₀ and asecond clock signal CK₁₈₀ used for controlling timing of the datastreams. In the example of FIG. 1B, the value of the odd data streamD_(ODD) becomes ready at the input of the multiplexing driver 10 beforea rising edge of the first clock signal CK₀, and the value istransferred to the full rate data stream D_(OUT) in response to a risingedge of the second clock signal CK₁₈₀. Additionally, the value of theeven data stream D_(EVEN) becomes ready at the input of the multiplexingdriver 10 before a rising edge of the second clock signal CK₁₈₀, and thevalue is transferred to the full rate data stream D_(OUT) in response toa rising edge of the first clock signal CK₀.

In the example of FIG. 1B, annotations for an example full data rate of32 gigabits per second (Gbps) are depicted. In this example, the halfrate data streams operate at 16 Gbps and a corresponding bit interval of62.5 picoseconds (ps), while the full rate data stream has a 31.25 psbit interval. The first clock signal CK₀ and the second clock signalCK₁₈₀ operate at 16 gigahertz (GHz) with a bit period offset from oneanother.

When operating at a full data rate of 32 Gbps, the 31.25 ps bit periodapproaches the process limit for certain processes, such as 16 nanometer(nm) processes associated with an inverter fan-out two delay of about 5ps.

To implement the multiplexing driver 10, the odd data stream D_(ODD)could be provided to a first inverter that drives a first T-gatemultiplexer, and the even data stream D_(EVEN) could be provided to asecond inverter that drives a second T-gate multiplexer. Additionally,the first T-gate multiplexer could pass the odd data stream D_(ODD) tothe output based on timing of the second clock signal CLK₁₈₀ (forinstance, in response to a rising edge), while the second T-gatemultiplexer could pass the even data stream D_(EVEN) to the output basedon timing of the first clock signal CLK₀ (for instance, in response to arising edge).

However, such an implementation has high output resistance due to aT-gate multiplexer being in series with each driver inverter.Furthermore, a large T-gate multiplexer size (to keep the outputresistance low due to the series combination of transistors) results inhigh parasitic capacitance. Moreover, including a series output resistorfor impedance matching (for instance, 50 Ohm) can further raise outputresistance and slow timing. Furthermore, asymmetries in the logic gatesused to drive the T-gate multiplexers leads to imbalances in rise/falltimes and a data eye that is bimodal.

In certain embodiments herein, the multiplexing driver 10 is implementedusing a mux-then-driver topology that provides multiplexing in apredriver circuit. Thus, the multiplexer is eliminated from the fullrate output path, thereby relaxing timing constraints to allow handlingof data streams of higher bit rates.

FIG. 2A is a schematic diagram of another embodiment of a multiplexingdriver 40. The multiplexing driver 40 includes a first driver halfcircuit 11, a second driver half circuit 12, an output resistor 13, andan output pin or pad 14.

As shown in FIG. 2A, the first driver half circuit 11 receives the evendata stream D_(EVEN), the first clock signal CK₀, and the second clocksignal CK₁₈₀, while the second driver half circuit 12 receives the odddata stream D_(ODD), the first clock signal CK₀, and the second clocksignal CK₁₈₀. The first driver half circuit 11 operates to provide theeven data stream D_(EVEN) to the output pad 14 based on timing of thefirst clock signal CK₀ (for instance, in response to a rising edge),while the second driver half circuit 12 operates to provide the odd datastream D_(ODD) to the output pad 14 based on timing of the second clocksignal CK₁₈₀ (for instance, in response to a rising edge).

Thus, the first driver half circuit 11 and the second driver halfcircuit 12 operate in an alternating or ping-pong sequence.

The first driver half circuit 11 and the second driver half circuit 12each include an output connected to the output pad 14 by way of theoutput resistor 13. Including the output resistor 13 can aid inachieving a desired output impedance match, for instance, 50 Ohms orother desired output impedance value. The output resistor 13 can beimplemented in a wide variety of ways including, but not limited to,using polysilicon or other resistive material having a geometry selectedto achieve a target resistance value. In certain implementations, theoutput resistor 13 is trimmable and/or otherwise controllable to achievean output resistance that provides compensation for variation.

In the illustrated embodiment, the first driver half circuit 11 includesa first driver p-type field effect transistor (PFET) 21, a first drivern-type field effect transistor (NFET) 22, a first pull-up predrivercircuit 23, and a first pull-down predriver circuit 24. Additionally,the second driver half circuit 12 includes a second driver PFET 31, asecond driver NFET 32, a second pull-up predriver circuit 33, and asecond pull-down predriver circuit 34. The predriver circuits areimplemented with multiplexing in accordance with the teachings herein.

Accordingly, the multiplexing driver 40 is advantageously implementedwith multiplexing that is implemented in predriver stages, therebyallowing a connection to the output pad 14 that goes through a lownumber of transistors.

For example, as shown in FIG. 2A, the first driver half circuit 11 candrive the output pad 14 logically high using the first driver PFET 21,which is connected in series with the output resistor 13 between a powersupply voltage V_(DD) (also referred to herein as a high supply voltage)and the output pad 14 with no other transistors therebetween. Likewise,the first driver half circuit 11 can drive the output pad 14 logicallylow using the first driver NFET 22, which is connected in series withthe output resistor 13 between a ground voltage (also referred to hereinas a low supply voltage or V_(SS)) and the output pad 14 with no othertransistors therebetween.

Accordingly, timing constraints are relaxed by eliminating a multiplexer(for instance, a T-gate multiplexer in cascade with a driver inverter)from the output resistance path.

The NFETs and PFETs can be implemented in a wide variety of ways. In oneexample, the multiplexing driver 40 is fabricated in a complementarymetal oxide semiconductor (CMOS) process, and the NFETs correspond ton-type metal oxide semiconductor (NMOS) transistors while the PFETscorrespond to p-type metal oxide semiconductor (PMOS) transistors.

FIG. 2B is a schematic diagram of another embodiment of a multiplexingdriver 50. The multiplexing driver 50 of FIG. 2B is similar to themultiplexing driver 40 of FIG. 2A, except that the multiplexing driver50 is implemented in a differential configuration.

For example, the multiplexing driver 50 includes a first non-inverted(+) driver half circuit 11 a, a second non-inverted driver half circuit12 a, a first output resistor 13 a, and a first output pad 14 a. Thefirst non-inverted driver half circuit 11 a operates to provide thenon-inverted even data stream D_(EVEN+) to the first output pad 14 a(which provides D_(OUT+)) based on timing of the first non-invertedclock signal CK₀₊, while the second non-inverted driver half circuit 12a operates to provide the non-inverted odd data stream D_(ODD+) to thefirst output pad 14 a based on timing of the second non-inverted clocksignal CK₁₈₀₊. The first non-inverted driver half circuit 11 a includesa first driver PFET 21 a, a first driver NFET 22 a, a first pull-uppredriver circuit 23 a, and a first pull-down predriver circuit 24 a,while the second non-inverted driver half circuit 12 a includes a seconddriver PFET 31 a, a second driver NFET 32 a, a second pull-up predrivercircuit 33 a, and a second pull-down predriver circuit 34 a.

With continuing reference to FIG. 2B, the first inverted driver halfcircuit 11 b operates to provide the inverted even data stream D_(EVEN−)to the second output pad 14 b (which provides D_(OUT−)) based on timingof the first inverted clock signal CK⁰⁻, while the second inverteddriver half circuit 12 b operates to provide the inverted odd datastream D_(ODD−) to the second output pad 14 b based on timing of thesecond inverted clock signal CK¹⁸⁰⁻. The first inverted driver halfcircuit 11 b includes a first driver PFET 21 b, a first driver NFET 22b, a first pull-up predriver circuit 23 b, and a first pull-downpredriver circuit 24 b, while the second inverted driver half circuit 12b includes a second driver PFET 31 b, a second driver NFET 32 b, asecond pull-up predriver circuit 33 b, and a second pull-down predrivercircuit 34 b.

Any of the driver circuits herein can be implemented differentially. Byimplementing a multiplexing predriver differentially, enhanced immunityagainst common-mode noise can be achieved.

FIG. 3A is a schematic diagram of one embodiment of a driver halfcircuit 70 for a multiplexing driver. For example, the driver halfcircuit 70 can connect to an output resistor 13 and an output pad 14 ofthe multiplexing driver as shown. FIG. 3B is one example of a timingdiagram for the driver half circuit 70 of FIG. 3A when operating at 32Gbps.

As shown in FIG. 3A, the driver half circuit 70 receives the even datastream D_(EVEN), the first clock signal CK₀, and the second clock signalCK₁₈₀. The driver half circuit 70 includes a driver PFET 21 (alsoreferred to as driver PFET M3), a driver NFET 22 (also referred to asdriver NFET M3′), a pull-up predriver circuit 63, and a pull-downpredriver circuit 64.

In the illustrated embodiment, the pull-up predriver circuit 63 includesa pull-down data NFET M1, a pull-up data PFET M2, a multiplexing NFETM4, and a pre-charge PFET M5. As shown in FIG. 3A, the pull-up predrivercircuit 63 controls activation of the driver PFET M3 at a node Y that ispre-charged to V_(DD) by the pre-charge PFET M5 when the first clocksignal CK₀ is low. Additionally, the pull-up data PFET M2 and thepull-down data NFET M1 control the node X to one of V_(DD) or ground(V_(SS)) based on a state of the even data stream D_(EVEN). In responseto the first clock signal CK₀ going high, the multiplexing NFET M4passes the value of node X to node Y to thereby control the driver PFETM3.

With continuing reference to FIG. 3A, the pull-down predriver circuit 64includes a pull-up data PFET M1′, a pull-down data NFET M2′, amultiplexing PFET M4′, and a pre-charge NFET M5′. As shown in FIG. 3A,the pull-down predriver circuit 64 controls activation of the driverNFET M3′ at a node Y′ that is pre-charged to ground by the pre-chargeNFET M5′ when the second clock signal CK₁₈₀ is high. Additionally, thepull-up data PFET M1′ and the pull-down data NFET M2′ control the nodeX′ to one of V_(DD) or ground based on a state of the even data streamD_(EVEN) In response to the second clock signal CK₁₈₀ going low, themultiplexing PFET M4′ passes the value of node X′ to node Y′ to therebycontrol the driver MFET M3′.

Thus, the pull-up predriver circuit 63 and the pull-down predrivercircuit 64 operate in a first phase associated with pre-charge followedby a second phase in which the output is pulled up or down based on astate of the even data stream D_(EVEN).

By implementing the driver half circuit 70 in this manner, a number ofperformance enhancements are achieved including, but not limited to, arelaxed setup time (t_(setup)).

Table 1 below provides a summary of operation of the pull-up predrivercircuit 63 over the first phase and the second phase.

TABLE 1 Phase CK₀ CK₁₈₀ D_(EVEN) node Y M4 M5 1 Low high settlingpre-charge off on to V_(DD) 2 High low transparent pull-down on off whenD_(EVEN) = l

FIG. 4A is a schematic diagram of another embodiment of a multiplexingdriver 100. FIG. 4B is one example of a timing diagram for themultiplexing driver 100 of FIG. 4A when operating at 32 Gbps. Themultiplexing driver 100 includes a first driver half circuit 70, asecond driver half circuit 80, an output resistor 13, and an output pad14.

The multiplexing driver 100 includes two half driver circuitsimplemented in accordance with the embodiment of FIG. 3A. For example,the multiplexing driver 100 includes the first half driver circuit 70,as described earlier with respect to FIG. 3A. The multiplexing driver100 further includes the second half driver circuit 80 used to controlthe output data stream Dom′ based on the odd data stream D_(ODD) andtiming of the first clock signal CK₀ and the second clock signal CK₁₈₀.

As shown in FIG. 4A, the second half driver circuit 80 includes apull-down data NFET M1″ (for pulling down node X″ when D_(ODD) is high),a pull-up data PFET M2″, a driver PFET M3″, a multiplexing NFET M4″(controlled by CK₁₈₀), a pre-charge PFET M5″ (for pre-charging node Y″to V_(DD) when CK₁₈₀ is low), a pull-up data PFET M1′″ (for pulling upnode X′″ when D_(ODD) is low), a pull-down data NFET M2′″, amultiplexing PFET M4′″ (controlled by CK₀), and a pre-charge NFET M5′″(for pre-charging node Y′″ to ground when CK₀ is high).

FIG. 5A is a schematic diagram of another embodiment of a multiplexingdriver 210. FIG. 5B is one example of a timing diagram for themultiplexing driver 210 of FIG. 5A.

With reference to FIG. 5A, the multiplexing driver 210 generates a fullrate data stream Dom′ by multiplexing a first quarter rate data streamDo, a second quarter rate data stream D₉₀, a third quarter rate datastream D₁₈₀, and a fourth quarter rate data stream D₂₇₀. Thus, themultiplexing driver 210 provides four-way interleaving.

In addition to receiving the quarter rate data streams, the multiplexingdriver 210 receives a first clock signal CK₀, a second clock signalCK₉₀, a third clock signal CK₁₈₀, and a fourth clock signal CK₂₇₀ thatare offset in phase from one another (by a bit interval of D_(OUT)).

In the example of FIG. 5B, annotations for operation at a full data rateof 56 Gbps are depicted. In this example, the quarter rate data streamsoperate at 14 Gbps and a corresponding bit interval of 71.8 ps, whilethe full rate data stream has a 17.8 ps bit interval. Furthermore, theclock signals operate at a 14 GHz with a bit period offset from oneanother.

By providing 4-way interleaving, higher output data rate can be achievedrelative to 2-way interleaving or no interleaving.

In certain embodiments herein, the multiplexing driver 210 isimplemented using a mux-then-driver topology that provides multiplexingin a predriver circuit. Thus, the multiplexer is eliminated from thefull rate output path to relax timing constraints.

FIG. 6 is a schematic diagram of another embodiment of a multiplexingdriver 260. The multiplexing driver 260 includes a first driver quartercircuit 211, a second driver quarter circuit 212, a third driver quartercircuit 213, a fourth driver quarter circuit 214, an output resistor215, and an output pin or pad 216.

As shown in FIG. 6 , the first driver quarter circuit 211 receives thefirst data stream Do, the second driver quarter circuit 212 receives thesecond data stream D₉₀, the third driver quarter circuit 213 receivesthe third data stream D₁₈₀, and the fourth driver quarter circuit 214receives the fourth data stream D₂₇₀. The driver quarter circuits211-214 are interleaved to drive the output pad 216 with theirrespective data streams based on timing of the first clock signal CK₀,the second clock signal CK₉₀, the third clock signal CK₁₈₀, and thefourth clock signal CK₂₇₀.

The driver quarter circuits 211-214 each include an output connected tothe output pad 216 by way of the output resistor 215. Including theoutput resistor 215 can aid in achieving a desired output impedancematch, for instance, 50 Ohms or other desired output impedance value.

In the illustrated embodiment, the first driver quarter circuit 211includes a first driver PFET 221, a first driver NFET 222, a firstpull-up predriver circuit 223 for controlling the first driver PFET 221,and a first pull-down predriver circuit 224 for controlling the firstdriver NFET 222. Additionally, the second driver quarter circuit 212includes a second driver PFET 231, a second driver NFET 232, a secondpull-up predriver circuit 233 for controlling the second driver PFET231, and a second pull-down predriver circuit 234 for controlling thesecond driver NFET 232. Furthermore, the third driver quarter circuit213 includes a third driver PFET 241, a third driver NFET 242, a thirdpull-up predriver circuit 243 for controlling the third driver PFET 241,and a third pull-down predriver circuit 244 for controlling the thirddriver NFET 242. Additionally, the fourth driver quarter circuit 214includes a fourth driver PFET 251, a fourth driver NFET 252, a fourthpull-up predriver circuit 253 for controlling the fourth driver PFET251, and a fourth pull-down predriver circuit 254 for controlling thefourth driver NFET 252.

The multiplexing driver 260 is advantageously implemented withmultiplexing in predriver stages, thereby allowing a connection to theoutput pad 216 that goes through a low number of transistors. Forexample, as shown in FIG. 6 , each of the driver quarter circuits211-214 can drive the output pad 216 to V_(DD) or ground (V_(SS))through a single transistor. Accordingly, timing constraints are relaxedby eliminating a multiplexer (for instance, a T-gate multiplexer incascade with a driver inverter) from the output resistance path.

FIG. 7A is a schematic diagram of one embodiment of a pull-up predrivercircuit 280 for a multiplexing driver. FIG. 7B is one example of atiming diagram for the pull-up predriver circuit 280 of FIG. 7A.

The pull-up predriver circuit 280 of FIG. 7A illustrates one embodimentof the pull-up predriver circuit 233 of FIG. 6 , and thus the pull-uppredriver circuit 280 can be included as part of a driver quartercircuit. As shown in FIG. 7A, connections of the pull-up predrivercircuit 280 to the second driver PFET 231 (also referred to as driverPFET M3), the output resistor 215, and the output pad 216 are depicted.

In the illustrated embodiment, the pull-up predriver circuit 280includes a first multiplexing NFET M1, a data NFET M2, a secondmultiplexing NFET M4, a first pre-charge PFET M5, and a secondpre-charge PFET M6.

As shown in FIG. 7A, the pull-up predriver circuit 280 controlsactivation of the driver PFET M3 at a node Y that is pre-charged toV_(DD) by the first pre-charge PFET M5 when the first clock signal CK₀is low and by the second pre-charge PFET M6 when the second clock signalCK₉₀ is low. The node Z is pulled low in response to the second clocksignal CK90 going high, and the data NFET M2 pulls the node X to groundwhen the data stream D₉₀ is also high. The value of node X is passed tonode Y through the second multiplexing NFET M4, which is controlled bythe first clock signal CK₀.

Thus, the pull-up predriver circuit 280 operates in multiple circuitphases. Table 2 below provides a summary of operation of the pull-uppredriver circuit 280 over the phases.

TABLE 2 Phase CK₀ CK₉₀ D₉₀ node Y M4 M1 1 Low low settling pre-chargeoff off after to V_(DD) transition (M5 + M6) 2 High low finishingpre-charge on off settling to V_(DD) (M6) 3 high high no changepull-down on on if D₉₀ = 1 4 Low high no change pre-charge off on toV_(DD) (M5)

The multi-phase circuit sequence is also graphically depicted in FIG.7B.

The pull-up predriver circuit 280 of FIG. 7A advantageously includes thefourth phase to turn-off the data NFET M4 and pre-charge node Y quickly(for example, as quickly as CK0 drops below the threshold voltage of thefirst pre-charge PFET M5) rather than waiting for the whole transistorsequence M1, M2, and M4 to turn off (in order).

Moreover, this pre-charge scheme provides fast performance time and/orlow parasitic capacitance while avoiding a need for a separatepre-charge transistor for node X (for instance an additional transistordirectly connected between V_(DD) and node X).

FIG. 8A is a schematic diagram of one embodiment of a driver quartercircuit 300 for a multiplexing driver. FIG. 8B is one example of atiming diagram for the driver quarter circuit 300 of FIG. 8A foroperation at 56 Gbps.

The driver quarter circuit 300 is depicted with connections to theoutput resistor 215 and the output pad 216. The driver quarter circuit300 includes a driver PFET 231 (also referred to as driver PFET M3), adriver NFET 232 (also referred to as driver NFET M3′), a pull-uppredriver circuit 280, and a pull-down predriver circuit 290.

The driver quarter circuit 300 is implemented with predriver circuitsimplemented in accordance with the embodiment of FIG. 7A. For example,the driver quarter circuit 300 includes the pull-up predriver circuit280 as discussed above with reference to FIG. 7A, as well as thepull-down predriver circuit 290 which corresponds to a complementaryversion of the pull-up predriver circuit 280 in which transistorpolarities and power supply connections are reversed and the clocksignals are delayed by 180 degrees to provide inversion.

As shown in FIG. 8A, the pull-down predriver circuit 290 includes afirst multiplexing PFET M1′ (controlled by CK₂₇₀ and connected betweenV_(DD) and node Z′), a data PFET M2′ (controlled by D₉₀ and connectedbetween node Z′ and node X′), a second multiplexing PFET M4′ (controlledby CK₁₈₀ and connected between node X′ and node Y′), a first pre-chargeNFET M5′ (controlled by CK₁₈₀ and connected between node Y′ and ground),and a second pre-charge NFET M6′ (controlled by CK₂₇₀ and connectedbetween node Y′ and ground).

The multi-phase circuit sequence of the driver quarter circuit 300 ofFIG. 8A is graphically depicted in FIG. 8B.

Drivers with Controllable Output Swing and Constant Output Impedance

In certain applications, such as SerDes, it is desirable for an outputdriver to have constant output impedance while at the same time havingcontrollable output swing to achieve desired signal amplitude. Forexample, implementing a driver with variable output amplitude controlallows for enhanced flexibility for achieving desired signal level.However, it is desirable for the change in output amplitude or swing tonot degrade performance by changing the output impedance from a desiredlevel.

Although current mode logic (CML) drivers can realize controllable swingand constant output impedance, CML drivers suffer from a number ofundesirable characteristics, such as high power consumption. Seriessource transistor (SST) drivers offer improved power performance, butsuffer from varying output impedance when the driver transistor size ischanged to adjust output amplitude.

SST drivers with controllable output swing and constant output impedanceare provided. In certain embodiments herein, a controllable driverincludes a group of differential SST driver slices that are connected inparallel with one another to drive a pair of output terminals providinga differential output signal, and a group of attenuator slices that areconnected in parallel with one another across the pair of outputterminals. Each attenuator slice can be implemented to have an on-stateresistance about equal to an on-state resistance of one of thedifferential SST driver slices. Additionally, the controllable driverincludes a control circuit that activates an attenuator slice for eachSST driver slice that is decommissioned to provide amplitude control.Thus, for every differential SST driver slice that is disabled foramplitude control, an attenuator slice is enabled.

Thus, the combined total number of active SST driver slices and activeattenuator slices remains constant, and the output impedance remains ata desired value (for instance, 50 Ohms).

In certain implementations, the control circuit can be implemented toalso disable any clock and data path circuits used to drive adifferential SST driver slice that is disabled for amplitude control. Byimplementing the controllable driver in this manner, any additionalcurrent draw by the attenuator array is outweighed by a currentreduction arising from disabling the clock and data path circuitsleading to the decommissioned slices.

FIG. 9 is a schematic diagram of one embodiment of a driver 400 withcontrollable swing and constant output impedance. The driver 400includes differential SST slices 401 a, 401 b, . . . 401 i operating inparallel with one another to drive a pair of output terminalsV_(OUT+)/V_(OUT−). The driver 400 further includes attenuator slices 402a, 402 b, . . . 402 j in parallel with one another across the pair ofdifferential output terminals V_(OUT+), V_(OUT−). The driver 400 furtherincludes a control circuit 403 and data/clock path slices 404 a, 404 b,. . . 404 i.

In the illustrated embodiment, the control circuit 403 generates a firstgroup of enable signals EN1 a, EN1 b, . . . EN1 i for enabling the SSTslices 401 a, 401 b, . . . 401 i, respectively. Additionally, thecontrol circuit 403 generates a second group of enable signals EN2 a,EN2 b, . . . EN2 j for enabling the attenuator slices 402 a, 402 b, . .. 402 j, respectively. The number of SST slices i and the number ofattenuator slices j can be the same or different. The control circuit403 maintains a total number of active SST driver slices and activeattenuator slices constant. Thus, for every differential SST driverslice that is disabled for amplitude control, an attenuator slice isenabled.

In certain implementations, each attenuator slice is implemented to havean on-state resistance about equal to an on-state resistance of one ofthe differential SST driver slices. For example, when operating at roomtemperature and nominal operating voltage, the on-state resistances canbe within 20% of one another, or more particularly within 5%, forexample, within 1%. Thus, the resistances of the attenuator slices anddifferential SST driver slices need not match exactly.

Although the attenuator slices and SST resistances can be implemented tobe about equal in resistance, other implementations are possible. Forexample, making the attenuator resistance greater can provide increasedamplitude control granularity.

In certain implementations, resistance tuning of the SST slices and/orattenuator slices can be provided to provide compensation for variation,for example, process, supply voltage, and/or temperature (PVT)variation.

Additionally or alternatively, the layouts and/or design topologies ofthe slices can be implemented such that the resistances of the SSTslices and attenuator slices track each other to account for variationarising from manufacturing and/or operating conditions.

When the on-state resistances are about equal and the combined totalnumber of active SST driver slices and active attenuator slices remainsconstant, the output impedance across attenuation settings remainsconstant at a desired value (for instance, 50 Ohms).

In certain implementations, the control circuit 403 can be implementedto also disable any clock and data path circuits used to drive adifferential SST driver slice that is disabled for amplitude control.For example, in the illustrated embodiment, the data/clock path slices404 a, 404 b, . . . 404 i also receive the enable signals EN1 a, EN1 b,. . . EN1 i.

FIG. 10A is a schematic diagram of another embodiment of a driver 500with controllable swing and constant output impedance.

The driver 500 is depicted as including [0 . . . n−m−1] number of activeSST slices 501 and corresponding data signals D and inverted datasignals DB. Each of the SST slices 501 includes a first driver PFET 511,a first driver NFET 512, a second driver PFET 513, a second driver NFET514, a first output resistor 515, and a second output resistor 516. TheSST slices 501 have an output resistance Rsst. The SST slices 501 drivea pair of output terminals (to generate a differential output voltage+Vout/−Vout) between which a first 50 Ohm load resistor 505 and a second50 Ohm load resistor 506 are connected.

With continuing reference to FIG. 10A, the driver 500 is depicted with[0 . . . m−1] number of active attenuator slices 502 connecteddifferentially across the pair of output terminals. Each of theattenuator slices 502 includes a first resistor 521, a second resistor522, and a T-gate multiplexer includes an NFET 523 and a PFET 524. Theattenuator slices have an output resistance Rsst′.

In this embodiment, n represents the number of SST slices desired fornominal amplitude at 50 Ohm, and m is the number of decommissioned SSTslices/activated attenuator slices.

FIG. 10B is a circuit diagram of a portion of the driver 500 of FIG.10A. The circuit diagram depicts a pair of resistors of resistance R1representing a transistor resistance of the SST slices. Additionally,the circuit diagram depicts a capacitor of capacitance Cx (which canincluded for any of the differential drivers herein for common modetermination) and a pair of resistors of resistance R2 representing theresistance of the attenuator slices. The diagram is annotated for asupply voltage (V_(DD)) of 1V and a ground voltage of 0V. In thiscircuit, the differential output voltage (V_(OUT+)−V_(OUT−)) of thedriver is equal to V_(DD)*(R2∥50 Ohm)/((R2∥50 Ohm)+R1).

FIG. 11 is a graph of one example of driver amplitude reduction versusattenuation setting for the driver 500 of FIGS. 10A and 10B. The x-axisrepresents the number of decommissioned SST slices and correspondingamplitude reduction. The output impedance is maintained at 50 Ohm acrossamplitude control settings.

FIG. 12A is a graph of one example comparison of driver output swingversus attenuation setting for two implementations of drivers. The graphincludes a plot 601 for one implementation of the driver 500 of FIGS.10A and 10B, and a second plot 602 for an array of selectable SST sliceswithout any attenuator slices.

FIG. 12B is a graph of one example comparison of driver current versusattenuation setting for two implementations of drivers. The graphincludes a plot 603 for one implementation of the driver 500 of FIGS.10A and 10B, and a second plot 604 for an array of selectable SST sliceswithout any attenuator slices.

FIG. 12C is a graph of one example comparison of driver output impedanceversus attenuation setting for two implementations of drivers. The graphincludes a plot 605 for one implementation of the driver 500 of FIGS.10A and 10B, and a second plot 606 for an array of selectable SST sliceswithout any attenuator slices.

With reference to FIGS. 12A to 12C, the total of SST slices andattenuator slices is selected to be 24 for the implementation of thedriver 500. Additionally, a 100 Ohm differential resistance issimulated. The results show a linear tradeoff between 1V AVDD currentreduction and smaller swing.

Moreover, with respect to FIG. 12A, the attenuation plot 601 followsFIG. 11 as expected while the attenuation plot 602 follows a more lineartrend due to a different attenuation method. Additionally, the currentplot 604 of FIG. 12B sees current increase as front-end stages (slices404 a, 404 b, . . . 404 i and slices 401 a, 401 b, . . . 401 i in FIG. 9) are still active while the attenuation plot 602 of FIG. 12A alsoinduces additional cross-bar current within the output driver stage.Furthermore, with respect to FIG. 12C, the lower value of outputimpedance plot 606 is due to the 24 slices used in the simulationinstead of 23 which would have increased the value by 4.3% (equal to1/23) to 97.25 Ohm to match the output impedance plot 605 closer.

FIG. 13 is a schematic diagram of one embodiment of a SerDes system 700.The SerDes system 700 includes a first semiconductor die 701 and asecond semiconductor die 702 connected over a high-speed link 703, whichcan be, for example, a pair of differential conductors.

The first semiconductor die 701 includes a serializer 704 that receivestwo or more incoming data streams of reduced bit rate relative to ahigh-speed data stream provided on the high-speed link 703. The secondsemiconductor die 702 includes a deserializer 705 that generates two ormore outgoing data streams of reduced bit rate based on the high-speeddata stream received from the serializer 704.

The serializer 704 includes a driver 706 implemented in accordance withone or more features of the present disclosure. For example, the driver706 can include predriver circuitry 707 that provides multiplexing ofthe incoming data streams and/or can include SST swing control 708 usingattenuator slices for output impedance control in accordance with theteachings herein.

CONCLUSION

The foregoing description may refer to elements or features as being“connected” or “coupled” together. As used herein, unless expresslystated otherwise, “connected” means that one element/feature is directlyor indirectly connected to another element/feature, and not necessarilymechanically. Likewise, unless expressly stated otherwise, “coupled”means that one element/feature is directly or indirectly coupled toanother element/feature, and not necessarily mechanically. Thus,although the various schematics shown in the figures depict examplearrangements of elements and components, additional interveningelements, devices, features, or components may be present in an actualembodiment (assuming that the functionality of the depicted circuits isnot adversely affected).

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the disclosure. Indeed, the novel apparatus, methods, andsystems described herein may be embodied in a variety of other forms;furthermore, various omissions, substitutions and changes in the form ofthe methods and systems described herein may be made without departingfrom the spirit of the disclosure. For example, while the disclosedembodiments are presented in a given arrangement, alternativeembodiments may perform similar functionalities with differentcomponents and/or circuit topologies, and some elements may be deleted,moved, added, subdivided, combined, and/or modified. Each of theseelements may be implemented in a variety of different ways. Any suitablecombination of the elements and acts of the various embodimentsdescribed above can be combined to provide further embodiments.Accordingly, the scope of the present invention is defined only byreference to the appended claims.

Although the claims presented here are in single dependency format forfiling at the USPTO, it is to be understood that any claim may depend onany preceding claim of the same type except when that is clearly nottechnically feasible.

What is claimed is:
 1. A driver circuit comprising: a pair of outputterminals configured to provide a differential output signal, the pairof output terminals comprising a first output terminal and a secondoutput terminal; a plurality of differential series source transistor(SST) driver slices electrically connected in parallel with one anotherand configured to drive the pair of output terminals, wherein eachdifferential SST driver slice comprises a first driver p-type fieldeffect transistor (PFET) connected between a high supply voltage and thefirst output terminal, a first driver n-type field effect transistor(NFET) connected between a low supply voltage and the first outputterminal, a second driver PFET connected between the high supply voltageand the second output terminal, and a second driver NFET connectedbetween the low supply voltage and the second output terminal; aplurality of attenuator slices connected in parallel with one anotheracross the pair of output terminals; and a control circuit configured toselectively deactivate one or more of the differential SST driver slicesto control an amplitude of the differential output signal, and to enablea corresponding number of the attenuator slices to provide outputimpedance compensation.
 2. The driver circuit of claim 1, wherein eachof the attenuator slices comprises a first resistor, a switch, and asecond resistor in series.
 3. The driver circuit of claim 2, furthercomprising a common-mode capacitor having one end connected between thefirst resistor and the second resistor.
 4. The driver circuit of claim2, wherein the switch comprises a p-type field effect transistor (PFET)and an n-type field effect transistor (NFET) in parallel.
 5. The drivercircuit of claim 1, wherein each attenuator slice has an on-stateresistance within 20 percent of an on-state resistance of one of thedifferential SST driver slices.
 6. The driver circuit of claim 1,wherein the driver circuit is operable in a plurality of attenuationsettings each associated with deactivating a different number of thedifferential SST driver slices.
 7. The driver circuit of claim 6,wherein the control circuit is configured to maintain a combined totalnumber of active differential SST driver slices and active attenuatorslices constant for each of the plurality of attenuation settings. 8.The driver circuit of claim 6, wherein an output impedance of the pairof output terminals is substantially constant across the plurality ofattenuation settings.
 9. The driver circuit of claim 1, furthercomprising a plurality of data and clock path slices coupled to theplurality of differential SST driver slices, wherein the control circuitis further configured to disable one or more of the plurality of dataand clock path slices when deactivating the one or more of thedifferential SST driver slices.
 10. A method of output swing control ina driver circuit, the method comprising: providing a differential outputsignal on a pair of output terminals, the pair of output terminalscomprising a first output terminal and a second output terminal; drivingthe pair of output terminals using a plurality of differential seriessource transistor (SST) driver slices electrically connected in parallelwith one another, wherein each differential SST driver slice comprises afirst driver p-type field effect transistor (PFET) connected between ahigh supply voltage and the first output terminal, a first driver n-typefield effect transistor (NFET) connected between a low supply voltageand the first output terminal, a second driver PFET connected betweenthe high supply voltage and the second output terminal, and a seconddriver NFET connected between the low supply voltage and the secondoutput terminal; deactivating one or more of the differential SST driverslices to control an amplitude of the differential output signal; andenabling a corresponding number of a plurality of attenuator slices toprovide output impedance compensation, wherein the plurality ofattenuator slices are connected in parallel with one another across thepair of output terminals.
 11. The method of claim 10, further comprisingproviding common-mode termination to the pair of output terminals usinga first resistor and a second resistor connected in series between thepair of output terminals and a common-mode capacitor having one endconnected between the first resistor and the second resistor.
 12. Themethod of claim 10, wherein each attenuator slice has an on-stateresistance within 20 percent of an on-state resistance of one of thedifferential SST driver slices.
 13. The method of claim 10, furthercomprising maintaining a combined total number of active differentialSST driver slices and active attenuator slices constant for each of aplurality of attenuation settings of the differential output signal. 14.The method of claim 10, further comprising maintaining an outputimpedance of the pair of output terminals substantially constant acrossthe plurality of attenuation settings.
 15. The method of claim 10,further comprising providing data to the plurality of differential SSTdriver slices using a plurality of data and clock path slices, anddisabling one or more of the plurality of data and clock path sliceswhen deactivating the one or more of the differential SST driver slices.16. A serializer/deserializer (SerDes) system comprising: adeserializer; and a serializer comprising a driver including a pair ofoutput terminals configured to provide a differential output signal tothe deserializer, a plurality of differential series source transistor(SST) driver slices electrically connected in parallel with one anotherand configured to drive the pair of output terminals, a plurality ofattenuator slices connected in parallel with one another across the pairof output terminals, and a control circuit configured to selectivelydeactivate one or more of the differential SST driver slices to controlan amplitude of the differential output signal, and to enable acorresponding number of the attenuator slices to provide outputimpedance compensation, wherein the pair of output terminals comprises afirst output terminal and a second output terminal, wherein eachdifferential SST driver slice comprises a first driver p-type fieldeffect transistor (PFET) connected between a high supply voltage and thefirst output terminal, a first driver n-type field effect transistor(NFET) connected between a low supply voltage and the first outputterminal, a second driver PFET connected between the high supply voltageand the second output terminal, and a second driver NFET connectedbetween the low supply voltage and the second output terminal.
 17. TheSerDes system of claim 16, wherein the driver circuit is operable in aplurality of attenuation settings each associated with deactivating adifferent number of the differential SST driver slices.
 18. The SerDessystem of claim 17, wherein the control circuit is configured tomaintain a combined total number of active SST driver slices and activeattenuator slices constant for each of the plurality of attenuationsettings.
 19. The SerDes system of claim 17, wherein an output impedanceof the pair of output terminals is substantially constant across theplurality of attenuation settings.
 20. The SerDes system of claim 17,wherein each of the attenuator slices comprises a first resistor, aswitch, and a second resistor in series, and a common-mode capacitorhaving one end connected between the first resistor and the secondresistor.